1. Field of the Invention
The present invention generally relates to mobile communication systems. More specifically, the present invention relates to mobile communication systems in which mobile stations can receive and demodulate signals transmitted on more than one frequency.
2. Description of the Related Art
FIG. 1 shows a general block diagram of an earlier mobile station 100 used in a cellular telephone system, such as a code division multiple access (CDMA) cellular telephone system. U.S. Pat. No. 5,109,390, which has been assigned to the assignee of the present invention and which is incorporated herein by reference, discloses a schematic diagram of an example of a CDMA cellular telephone system and a block diagram of a mobile station used in such a system. Referring to FIG. 1, mobile station 100 comprises an antenna 105 for communicating with base stations, a transmitter 110 for transmitting signals from the mobile station 100, a receiver 120 for receiving signals, a searchers and demodulators unit 130, and a duplexer 115 coupled to the antenna 105, transmitter 110, and receiver 120 for properly routing outgoing signals from the transmitter 110 to antenna 105 and incoming signals from antenna 105 to receiver 120. Duplexer 115 is a conventional duplexer that may be a full duplexer, which allows transmitting and receiving signals simultaneously, or a half duplexer, which allows for only one of transmitting or receiving signals at any given time.
Receiver 120 comprises a first band pass filter (BPF) 121 coupled to duplexer 115, a mixer 122 coupled to the first BPF 121, a local oscillator (LO) 123 coupled to mixer 122, a second BPF 124 coupled to mixer 122, and a subreceiver 125 coupled to the second BPF 124. Subreceiver 125 includes a frequency translator 126, which may be either a digital or analog frequency translator, and a third BPF 127.
Duplexer 115 routes incoming signals to the first BPF 121 which in turn sends a band passed version of the incoming signals to mixer 122. Mixer 122 also receives a second input from LO 123. The output of mixer 122 is sent to the second BPF 124 which sends a band passed version of its input signal to subreceiver 125. Frequency translator 126 receives the output of the second BPF 124 and shifts the received signal in the frequency domain such that its output is centered around a desired frequency, namely the frequency on which signals are carried as they are transmitted between the mobile station 100 and a base station with which the mobile station 100 is in communication. The third BPF 127 receives the output of frequency translator 126 and outputs a band passed version of its input. The third BPF 127 has a band pass of 1.25 MHz and is centered around the frequency on which signals are carried as they are transmitted between the mobile station 100 and the base station with which the mobile station 100 is in communication. The output of third BPF 127 is transmitted to the searchers and demodulators unit 130. The demodulators in searchers and demodulators unit 130 demodulate the signals on incoming waveforms. Thereafter, the demodulation fingers of the demodulators remove the codes of a communication code channel, such as Walsh codes and pseudorandom noise (PN) codes, from the demodulated signals, and combine the removed codes. The searchers in the searchers and demodulators unit 130 search for the existence of a structured waveform, such as codes of a communication code channel, e.g., Walsh codes or PN codes. Examples of searchers and demodulators are disclosed in U.S. Pat. Nos. 5,103,459, 5,490,165, and 5,506,865, all of which have been assigned to the assignee of the present invention and are incorporated herein by reference. It is to be noted that in some of the above referenced patents, a digital receiver or a digital data receiver may refer to a demodulator or a combination of a searcher and demodulator(s). Similarly, an analog receiver may refer to what in the present application is referred to as receiver 120 or an equivalent thereof.
As the subreceiver is tuned to only one frequency at any given time, the mobile station can be in communication only with a base station transmitting signals on the frequency range to which the mobile station is tuned. This limitation with respect to the frequency to which the mobile station is tuned causes the mobile station and the wireless communication system within which the mobile station operates to suffer from several disadvantages. First, the mobile station cannot be in soft handoff between two different frequencies. Second, the mobile station cannot monitor or search for pilots at more than one frequency at any given time. Third, in the idle state, the mobile station cannot monitor or search for pages at more than one frequency at any given time.